After starting, combustion engines desire to warm up rapidly to reduce fuel consumption and keep pollutant emissions low. Recirculation of exhaust gas, also referred to as exhaust gas recirculation (EGR), is an efficient method of assisting the heating of the combustion engine after starting. In this case, the exhaust gas is passed from the exhaust tract, by an EGR system, into the intake tract of the combustion engine. The EGR system may comprise a cooler device for cooling the exhaust gas. This cooler device may not be operated continuously, e.g. if the exhaust gas is supposed to maintain its temperature. For example, the cooler device may be disabled or bypassed during the engine start where an engine temperature is less than a threshold temperature (e.g., a cold-start). However, even if it is not being operated, the cooler device has a thermal mass which absorbs heat from the exhaust gas. For this reason, a bypass duct, by means of which exhaust gas can be diverted passed the cooler device may be arranged in the EGR system. The bypass duct has a smaller thermal mass than the cooler device, ensuring that the exhaust gas releases less heat when it is passed through the bypass duct. Under starting conditions however, while the walls of the bypass duct are still cold, the exhaust gas also releases heat to the material of the bypass duct.
With increasing time in operation of the combustion engine, the EGR system and hence also the casing of the bypass duct can be greatly heated by exhaust gas, making it desirable to apply cooling to protect the housing from excessive heating. Depending on the operating state, there are various thermal demands on a bypass duct configuration. Under starting conditions, thermally insulating the bypass duct may limit heat loss to the environment. As the time in operation increases, however, an increasing amount of heat from the exhaust gas is generally also transferred to the material of the bypass duct, even if it flows through the cooler device and not directly through the bypass duct. It is therefore the object of the present disclosure to provide a thermal insulation for the bypass duct which can also be used as thermal protection for the material of the bypass duct.
In one example, the issues described above may be addressed by an EGR system in a motor vehicle for passing exhaust gas out of an exhaust tract into an intake tract of a motor vehicle, said system having a duct with a cooler device and a bypass duct, in which the bypass duct is bounded in a radial direction by a double wall with a cavity which is in fluid connection in each case via at least one opening in an outer wall of the double wall with a first flow circuit and a second flow circuit and which can be filled with gas or liquid to control the temperature of the bypass duct.
In this way, the system allows both thermal insulation and cooling or heating of the bypass duct, depending on the operating situation. For thermal insulation of the bypass duct, the cavity may be filled with gas to restrict heat loss from the recirculated exhaust gas. The cooling and heating of the bypass duct is dependent on the temperature of the fluid medium, (e.g., a liquid), in particular, a liquid coolant, relative to the temperature of the exhaust gas. The bypass duct is cooled when the fluid medium is warmer than the exhaust gas. Cooling may be performed to avoid overheating of the bypass duct. Moreover, it is possible, via both the thermal insulation and the heating of the bypass duct, to control the temperature of the bypass duct in such a way that the exhaust gas releases as little heat as possible or that heat is fed to the exhaust gas. To heat the bypass duct, the fluid medium has a temperature which is higher than the temperature of the exhaust gas. The fluid medium can be used, in particular, for heating when it has not yet cooled after absorbing heat from the exhaust gas and is warmer than cool exhaust gas, which is formed in the starting phase and in low-load phases of the combustion engine, for example. The exhaust gas heats up during this process and, in addition to counteracting condensation, there is the advantageous effect that the combustion engine reaches an operating temperature more quickly or does not cool down too much below said temperature. Moreover, thermal insulation or heating has the advantageous effect that as little as possible water contained in the exhaust gas condenses, water which, during an operating phase in which no exhaust gas is being recirculated and an exhaust gas recirculation valve in the EGR system is closed, could agglomerate into large droplets which enter the compressor of a turbocharger when the EGR valve is opened and could cause damage due to droplet impact. The EGR system is a low-pressure EGR system, in some examples, but may also be a high-pressure EGR system without departing from the scope of the present disclosure.
The term “flow circuit” refers to an arrangement of devices in which a fluid medium, e.g., a gas or a liquid, can flow and the flow of the medium is controlled. The flow circuit may or may not comprise a closed circuit for the medium. It is also possible for different media to flow in a flow circuit.
In the system according to the present disclosure, the first flow circuit has at least one first line with at least one first valve and at least one second line with at least one second valve. The lines allow the cavity to be filled with gas and liquid to be evacuated from the cavity while it is being filled with gas. As gas, it is possible to use air or some other suitable gas, for example, and, as liquid, to use water or some other liquid suitable as a cooling liquid.
At least one pump is arranged in the first flow circuit of the system. The pump is used to evacuate the liquid from the cavity in the double wall of the bypass duct. A pump, which is used particularly to pump the liquid into the cavity, is likewise arranged in the second flow circuit.
The first flow circuit of the system comprises a container, in which there is a gas in a first subregion and a liquid in a second subregion. Here, the gas is provided to fill the cavity, and the liquid is supplied from the cavity. The use of the container is may monitor that the gas volume introduced corresponds to the liquid volume discharged as the gas in the cavity in the common container is replaced by liquid.
It is also possible for the first flow circuit of the system to comprise a separate gas reservoir. The gas reservoir is a pressurized gas container, e.g. a compressed air cylinder, wherein the gas used is air, in one example. In this embodiment, the first flow circuit has a separate first liquid reservoir. The first liquid reservoir is used to receive a liquid evacuated from the cavity. In this case, the first liquid reservoir may be integrated with the gas reservoir in a single unit.
In the system, the second flow circuit comprises at least one third line with at least one third valve and at least one fourth line with at least one fourth valve.
The second flow circuit further comprises a second liquid reservoir. A liquid can flow from the second liquid reservoir, via the third line, into the cavity and from the cavity, via the fourth line, back into the second liquid reservoir. The second flow circuit is thus a closed flow circuit. Ideally, the second flow circuit likewise has a pump for producing a flow. It is possible for the first liquid reservoir to be connected to the second liquid reservoir to feed liquid evacuated from the cavity during filling with gas back to the second circuit.
A first method for controlling the temperature of exhaust gas recirculated through the bypass duct of the EGR system, wherein the cavity is filled with a gas or a liquid depending on the operating situation is described in greater detail below
Specifically, a controller with instructions stored thereon that when executed enable the controller to carry out thermal insulation of the bypass duct, which includes closing the third and fourth valves, opening the first and second valves, evacuating liquid from the cavity via the second line while simultaneously filling the cavity with gas via the first line, and closing the first and second valves. In the method, the initial situation is one in which the cavity is initially filled with a liquid or in which at least a volume of liquid is present in the cavity, said liquid being removed from the cavity as gas flows into the cavity. This can be the case under starting conditions, for example, wherein liquid from a previous operation of the system is still present in the cavity. It is furthermore possible, by means of the method, to transfer the bypass duct during operation from a cooling mode, in which the material of the bypass duct and of a housing surrounding the bypass duct are protected from excessive heating, to a thermal insulation mode, in which the exhaust gas temperature is maintained as far as possible.
The controller further includes instructions stored thereon that when executed enable the controller to carry out a second method to cool the bypass duct, where the second method includes closing the first and second valves, opening the third and fourth valves, and evacuating the gas from the cavity via a gas valve while simultaneously introducing into the cavity a liquid which is cooler than an exhaust gas passed through the bypass duct, said liquid flowing at a constant rate from the third line, through the cavity, into the fourth line.
In the additional steps, the material of the bypass duct may cool if it overheats with increasing time in operation of the combustion engine. If the bypass duct is to be thermally insulated again at another, later time, e.g. in an operating state with cooler exhaust gas, the controller may switch from operating the second method to initiating the first method. It will be appreciated that the controller may also switch from the first method to the second method when desired. It is thus possible to switch between thermal insulation, heating and cooling of the bypass duct, depending on requirements or the operating state.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.